The reaction between myoglobin and the lipid hydroperoxide 13(S)-hydroperoxy-9,11(cis,trans)-octadecadienoic acid (HPODE) was studied kinetically by spectrophotometric, polarographic and analytical methods. Metmyoglobin catalysed the decomposition of HPODE, resulting in peroxide, oxygen and conjugated diene depletion, together with the transient production of ferryl myoglobin. The reaction stoichiometry was 2:1:1 for peroxide to oxygen to conjugated diene, whereas the myoglobin remained generally intact. This stoichiometry and the rates of change of conjugated diene and ferryl myoglobin concentrations were not completely consistent with previously proposed mechanisms. We propose a novel mechanism in which HPODE reacts with both ferric myoglobin and ferryl myoglobin to form a redox cycle. Both peroxyl and alkoxyl radicals are produced, explaining the observed stoichiometry of peroxide, oxygen and conjugated diene depletion and the transient appearance of ferryl myoglobin. Computer simulation shows that this mechanism is fully capable of reproducing the observed time courses of all components.

Pseudomonas aeruginosa is an opportunistic pathogen requiring iron for its survival and virulence. P. aeruginosa can acquire iron from heme via the nonredundant heme assimilation system and Pseudomonas heme uptake (Phu) systems. Heme transported by either the heme assimilation system or Phu system is sequestered by the cytoplasmic protein PhuS. Furthermore, PhuS has been shown to specifically transfer heme to the iron-regulated heme oxygenase HemO. As the PhuS homolog ShuS from Shigella dysenteriae was observed to bind DNA as a function of its heme status, we sought to further determine if PhuS, in addition to its role in regulating heme flux through HemO, functions as a DNA-binding protein. Herein, through a combination of chromatin immunoprecipitation–PCR, EMSA, and fluorescence anisotropy, we show that apo-PhuS but not holo-PhuS binds upstream of the tandem iron-responsive sRNAs prrF1,F2 . Previous studies have shown the PrrF sRNAs are required for sparing iron for essential proteins during iron starvation. Furthermore, under certain conditions, a heme-dependent read through of the prrF1 terminator yields the longer PrrH transcript. Quantitative PCR analysis of P. aeruginosa WT and Δ phuS strains shows that loss of PhuS abrogates the heme-dependent regulation of PrrF and PrrH levels. Taken together, our data show that PhuS, in addition to its role in extracellular heme metabolism, also functions as a transcriptional regulator by modulating PrrF and PrrH levels in response to heme. This dual function of PhuS is central to integrating extracellular heme utilization into the PrrF/PrrH sRNA regulatory network that is critical for P. aeruginosa adaptation and virulence within the host.

No abstract

No abstract

Forkhead box P (FoxP) are members of the human Fox family of transcription factors (FoxA-FoxS), involved in diverse processes such as intellectual and immune system regulation. The four FoxP subfamily members share the unique ability to dimerize via domain swapping using their DNAbinding domain. Specifically, we study FoxP1 and FoxP2 dimerization due to its impact on various human diseases. Both proteins share 88% sequence identity but have distinct dimerization properties, with K D values ranging from mM (FoxP1) to mM (FoxP2). However, the structural and energetical properties of heterodimerization and the role of the DNA-binding domain and its DNA ligand in this association are unknown. In this context, we integrated single-molecule FRET and biophysical studies to probe the FoxP1-FoxP2 heterodimerization in vitro, emphasizing the effect that the ligand DNA has on the heterodimerization and the heterodimer's structural dynamics. Biophysical assessment of the heterodimerization indicated that the K D ascertained is similar to the FoxP1 homodimerization, and therefore dramatically different from the FoxP2 homodimerization value. Singlemolecule FRET analysis of FoxP1 homodimer and heterodimer (FoxP1:-FoxP2) showed that the heterodimer increases the local dynamics and the accumulation of an expanded intermediate, suggesting that FoxP2 presumably has evolved to heterodimerize instead of homodimerizing. Finally, DNAbinding assays indicated that monomer:DNA complex decreases the heterodimerization affinity, whereas having no substantial effect in the heterodimer's stability. These findings highlight relevant aspects of the regulatory role of DNA in the adoption of heterodimers and, therefore, in their gene-related expression.

No abstract

Pseudomonas aeruginosa is an opportunistic pathogen requiring iron for its survival and virulence. P. aeruginosa can acquire iron from heme via the heme assimilation system (Has) and Pseudomonas heme uptake (Phu) systems. The Has and Phu systems have non-redundant roles in heme sensing and transport, respectively. However, despite their respective roles heme taken up by either the Has or Phu system is regulated at the metabolic level by the cytoplasmic heme binding protein PhuS, which controls heme flux through the iron-regulated heme oxygenase HemO. Herein, through a combination of CHIP-PCR, EMSA and fluorescence anisotropy we show PhuS binds upstream of the tandem iron-responsive sRNAs prrF1,F2. Furthermore, qPCR analysis of the PAO1 WT and ΔphuS allelic strain shows loss of PhuS abrogates the heme dependent regulation of PrrH. Taken together our data shows PhuS, in addition to its role in regulating extracellular heme metabolism also functions as a transcriptional regulator of the heme-dependent sRNA, PrrH. This dual function of PhuS is central to integrating extracellular heme utilization into the PrrF/PrrH sRNA regulatory network critical for P. aeruginosa adaptation and virulence within the host.